The present invention relates to a liquid crystal display device, and in particular, to a circuit for driving a matrix liquid crystal display device.
Matrix liquid crystal displays are known in the art. Reference is made to FIGS. 1 through 3 in which a conventional matrix liquid crystal display is provided. A liquid crystal panel generally indicated as 1 is composed of a liquid crystal layer 5, a first substrate 2 and a second substrate 3 for sandwiching the liquid crystal layer 5 therebetween. A plurality of common electrodes Y1 through Y6 are oriented on substrate 2 in the horizontal direction and a plurality of segment electrodes X1 through X6 are formed on substrate 3 in substantially the vertical direction to form a matrix. Each intersection of common electrodes Y1 through Y6 and segment electrodes X1 through X6 forms a display dot 7. Display dots 7 marked by the hatching indicate an ON state, and the blank dots 7 indicate an OFF state. The dot structure of liquid crystal panel 1 is limited to a six by six matrix for simplicity however, in exemplary embodiments the number of dots of liquid crystal panel 1 may be much greater.
The voltage standard method is conventionally used for driving the prior art matrix liquid crystal display device. A selected voltage or non-selected voltage is sequentially applied to each of common electrodes Y1 through Y6. The period required to apply the successive selected voltage or non-selected voltage to all the common electrodes Y1 to Y6 is one frame.
Simultaneous to the successive application of the selected voltage or non-selected voltage to each common electrodes Y1 through Y6, an ON voltage or OFF voltage is applied to each segment electrode X1 through X6. Accordingly, to turn a display dot 7, the area in which one common electrode intersects one segment electrode, to the ON state, an ON voltage is applied to a desired segment electrode when the common electrode is selected by providing a selected voltage to the desired common electrode. Similarly if the display dot is turned OFF, the OFF voltage is applied to the desired segment electrode.
Reference is now also made to FIGS. 2 and 3 in which examples of the actual driving waveforms (waveform of the applied voltage) applied at the electrodes are provided. FIG. 2A shows the segment voltage waveform applied to segment electrode X5 over time. FIG. 2B shows the common electrode waveform applied to common electrode Y3 over time. FIG. 2C shows the voltage waveform applied for producing the ON state at display dot 8, the intersection of segment electrode X5 and common electrode Y3.
FIG. 3A shows the segment voltage waveform applied to segment electrode X5 over time. FIG. 3B shows the common voltage waveform applied to common electrode Y4 over time. FIG. 3C shows the voltage waveform applied to the display dot at the intersection of segment electrode X5 and common electrode Y4 to produce the OFF state.
In FIGS. 2 and 3, F1 and F2 indicate the frame period.
______________________________________ During frame period F1, selected voltage = V0, non-selected voltage = V4 ON voltage = V5, OFF voltage = V3 During frame period F2, selected voltage = V5, non-selected voltage = V1 ON voltage = V1, OFF voltage = V2, ______________________________________
wherein; EQU V0-V1=V1-V2=V EQU V3-V4=V4-V5=V EQU V0-V5=n V
(n is a constant).
Accordingly, by changing the polarity of the voltage which is applied to display dots 7 during frame periods F1 and F2, alternating driving is accomplished. It follows that whether the display dot 7 is ON or OFF depends on whether the ON voltage or OFF voltage is applied to the desired segment electrode when the selected voltage is applied to the intersecting common electrode corresponding to the desired display dot. This driving method is the voltage standard means used in the prior art.
The prior art structure and driving method has been less than satisfactory. When matrix liquid crystal display 1 is driven by the above conventional voltage standard method, the uniform rectangular waveforms illustrated in FIGS. 2 and 3 are not actually applied to display dots 7. Distortions in the applied waveforms occur. A first reason for the distortion is that each display dot 7 has an inherent electrical capacity based on the area of each dot 7, the thickness of the liquid crystal layers, the dielectric constant of the liquid crystal materials and so on. Secondly, both the common electrode and segment electrode are formed of a transparent conductive film having a surface resistance of about several tens of ohms as well as fixed electrical resistance. Therefore, even if the uniform rectangular waveforms as shown in FIGS. 2 and 3 are applied by the driving circuit, the waveform which is actually applied to the display dots becomes deformed and cross talk results. As a result, it becomes necessary to generate the difference of the effective voltage of the waveform which is applied to each display dot, resulting in the generation of contrast cross talk.
Observation has demonstrated that deformation of the voltage waveform being applied to the display dots occurs based upon relationship dependent on the pattern of the characters or drawings which is displayed by the liquid crystal display device. Secondly, the change of the effective voltage based on the deformation of the voltage waveform which is applied to the display dots causes the contrast crosstalk.